Update:
Right now i'm trying to sort out the stats of all the creatures in one big list. I've quickly come to realize that judging their danger level with DFs combat system is fairly difficult looking at numbers alone. For the next while i'm going to have to run lots of tests with modded wolves and cougars (easy to find in adventure mode) so I get get a better feel for the impact of different numbers.
In the meantime i've had some thoughts on other things:
-The closest thing to Elbereth in DF is mass traps and moated bridges. I can't do much against the moated bridges, so any creature that is not affected by Elbereth (@'s) will have trap avoid. The bad news is, Master Kaen is a @ and there are no wands of sleep in DF. The good news is you'll have a way to defend yourself against the abomination of evil known as Demogorgon.
-Like horses I think i'll take out large dogs and have 'dog' be it's grown form. War Large Dog would sound silly
. Also I may leave the stats of vanilla dogs alone. If they were to be as strong as nethack dogs there would be no need to have soldiers.
-There are over 40 individual monsters in Nethack that are worthy of megabeast status, 28 of which are unique and
need to be megas. I may have to demote some of the less important dragons to semi-mega status.
-The power monster. I'll be removing out all DF monsters that I can afford to take out, and that includes Demons. Suprisingly there are very few creatures that can take it's place once you remove unique monsters as candidates and take HFS into account. I've narrowed it down to Arch-liches or a high priest called High Priest of Moloch. Please let me know which you'd prefer